Inductive proximity sensors are noncontact devices that sense the presence of target objects. Changes in the coil system impedance are caused by disturbances in the generated magnetic field that are due to the presence of a conductive and/or magnetic target object in a sensing area. The coil system impedance changes are sensed and an output signal is generated according to the sensed impedance to indicate the presence or absence of a target object in the sensing area. The coil is located in a housing with a sensing face from which the magnetic field extends. The housing is typically a cylindrical barrel shape, a rectangular structure, a slot, or a ring. Cylindrical sensors are often threaded to allow flush or non-flush mounting to a structure with a threaded hole, with the sensing face at one end of the barrel being carefully spaced from the path along which the target object travels. Slot style sensors detect the presence of a target as it passes through a sensing slot, typically in the form of a “U”-shaped channel. Ring shaped sensors sense objects passing through the center of the ring body. The coil system may be fashioned in a variety of arrangements, including single coil types with an excitation generator, where the proximity of a conducting/magnetic target changes the coil inductance. Dual coil arrangements are possible with two coils constituting a differential transformer with an excitation generator, where the presence of the target changes the coupling level and inductance of the coils. Tank circuit type inductive sensors include a single coil and a capacitor arranged as resonant tank circuit, wherein the presence of a target changes the coil inductance and the impedance of resonant tank. Other forms of inductive sensors may combine two or more of these circuit topologies.
In metal face sensors, the sensor circuitry including the coil system is enclosed within a housing made from stainless steel or other corrosion-resistant materials to avoid exposure to dirt, dust, humidity, etc. so as to mitigate sensor degradation. The sensing face, moreover, is susceptible to unintended impact by moving target objects being sensed. Metal-face inductive proximity sensors include a metal face structure between the coil system and the external sensing area to protect the coil and associated ferrite core from the external conditions including contaminants and impact damage, where the face and the rest of the housing structure may form a single integrated enclosure. However, since the sensing field generated by the coil system passes through the sensing face, conventional metal face inductive proximity sensors suffer from reduced sensing distances due to depletion of the sensing field before it reaches the external sensing area. Plastic faced inductive proximity sensors have been introduced, in which the sensing face is made of plastic to mitigate field depletion. These sensor face designs, however, do not provide as much protection for the coil system from extreme ambient conditions and impacts. Thus, there remains a need for improved metal face inductive proximity sensors by which the protection advantages of the metal sensing face can be achieved while mitigating the adverse sensing distance limitations associated with conventional metal face designs.